(CBS/AP) TUCSON, Ariz. - Polly Klaas. Elizabeth Smart. Megan Kanka. The names are synonymous with a parent's worst nightmare: a child snatched by a stranger from the safety of her own home.
Now, police in Tucson, Ariz., are trying to determine what happened to 6-year-old Isabel Mercedes Celis. Her parents say they awoke on Saturday to find her missing. Police said a window was open with the screen pushed aside.
While officers are investigating all possibilities in her disappearance, experts say, abduction from the home is relatively rare, with just over 18 children taken each year.
"It's unusual, but it's not unprecedented," said Ernie Allen, president of the National Center for Missing and Exploited Children, which is involved in the search.
Each year, 58,000 children are abducted by strangers and released, according to the most recent statistics. Of those, 115 were "stereotypical" kidnappings carried out by strangers who either killed the children or held them for ransom. And 16 percent of those were taken from home.
"Lots of children disappear from home. Those investigations usually find some family involvement," he said. "In this case, (and in the Elizabeth Smart and Polly Klaas cases), what you saw was sometimes it really does happen this way." (Watch Miller's analysis at left.)
Nearly three quarters of the victims are girls, and 38 percent of them are 12 to 14. At 24 percent, the second largest victimized group is the one Isabel belongs to: girls ages 6 to 11.
In Tucson, the possibility that a kidnapper could be in their midst unnerved some parents.
"I put two-by-fours in their windows this morning," said Erin Cowan, who has worked with Isabel's mother at Tucson Medical Center and has a daughter, 7, and son, 12. "I guess you can't be too careful, sadly."
Since Saturday, investigators and volunteers fanned across Isabel's neighborhood and an area landfill searching for clues. Volunteers posted fliers with a photo of Isabel about 4-feet-tall with brown hair and hazel eyes holding a school award.
Her parents, identified by friends as Becky and Sergio Celis, told investigators they last saw the first-grader at 11 p.m. Friday. Her mother, a nurse, was at work Saturday when her father went to wake her at 8 a.m. and discovered her missing, police said.
Police call the case a "suspicious disappearance/possible abduction."
"We're not ruling anything out of the investigation at this point because we really need to keep our mind open about all the information that's been brought to us," police Chief Roberto Villasenor said.
On Monday, FBI dogs one that can find human remains and the other used for search and rescue went through the home and turned up information that required a follow-up, but police declined to say what that was.
Officers are also interviewing sex offenders in the area. It has become standard practice for all abduction investigations.